1. Field of the Invention
The present invention relates to an automatic frequency control circuit and, more particularly, to an automatic frequency control circuit for correcting the influence of frequency offset in a radio data communication terminal in a scheme in which transmission path characteristics are obtained during the preamble period by narrow-band modulation such as GMSK to equalize the received signal.
2. Description of the Prior Art
In many cases, a synthesizer, a quartz oscillator, or the like having high oscillation precision cannot be used to generate a radio frequency due to limitations on cost, size, and the like. In general, therefore, a radio unit performs automatic frequency control, and communication is performed while controlling any frequency offset between the transmission side and the reception side.
FIG. 1 shows an example of a conventional automatic frequency circuit of an FM receiver.
The FM receiver comprises an antenna 41 for receiving a radio wave transmitted from the transmitter side, a bandpass filter 42 for extracting a signal having a necessary band from the received signal, an amplifier 43 for amplifying the level of the signal passing through the bandpass filter 2 to a necessary level, a mixer 44 for decreasing the frequency of the received signal to a necessary frequency, an oscillator 45 for generating a necessary frequency, an amplifier 46 for amplifying the level of the received signal to a necessary level, a limiter 47 for limiting the amplitude of the received signal, and a frequency discriminator 48 for demodulating the received signal.
After a signal received by the antenna 41 is amplified by the amplifier 43, it is changed by the mixer 44 to a signal having a necessary frequency, amplified by the amplifier 46, limited in amplitude by the limiter 47, and demodulated by the frequency discriminator 48. The DC component of the demodulated data changes its polarity to positive or negative depending on the offset direction of the center frequency from the center of the S-mode characteristic of the discriminator. This DC component is negatively fed back to control the frequency of the oscillator.
In the prior art, an error undesirably occurs in frequency control because the frequency spectrum changes or becomes asymmetrical in the frequency selection multi-path fading environment.